Snapper Zero Turn blades/belt issue

We have a Snapper Zero turn Model # NZMJ25611KH. When we engage the blades, they sound like they are turning full speed. When we start to mow, they slow down significantly. I have replaced the belt but I'm not sure if I routed it correctly. Is there a spring that needs to be tightened? Any suggestions or thoughts would be appreciated.
 
Scot